Looking at the circuits and layout of components in both tuners, it looks like
MFJ is using transformer to feed the balanced line, while Palstar has balanced
circuit feeding the balanced line.
Balanced line exhibits fluctuation of voltage and current along its length
(standing wave circuit), depending on the frequency used - it could have high
voltage at its end if the electrical length is close to the quarter wave
multiples. Ideally (open) balanced line feeders should have electrical length
equal or close to half wave multiples, it would reflect impedance at the
antenna feedpoint.
Adding some feeder length could alleviate the problem (pull away extra feeder
with some string or wind it on plastic tube).
MFJ has one winding of the balance transformer grounded and this could be
helping to shift the current/voltage distribution along the circuit and prevent
high voltage at the terminals and minimizes the danger of arcing.
I think Paul/Palstar might appreciate the problem, in case the tuner was tested
to resistive dummy rather than extreme real life situations.
GL, Yuri, K3BU.us
----- Original Message -----
From: Paul Mackanos - K2DB
Date: Monday, March 12, 2012 10:00 am
Subject: [CQ-Contest] Looking for input on balanced line feeder length
To: cq-contest@contesting.com
> Forward from one of the guys in our club.
>
> Paul K2DB
>
>
>
> Hi all,
>
>
>
> I am looking for any input anyone has in regard to balanced
> line feeder
> length. I am sure most are familiar with my situation with my
> antenna tuner.
>
>
>
>
> I have a 135 foot dipole, fed with approximately 53 feet of
> 450 ohm
> ladder line.
>
>
>
> My original tuner was an MFJ-986 and it has worked without and
> problems with
> the amp @ 1k.
>
>
>
> I wanted to changed to a true balanced line tuner, I did my
> research and
> found the the Palstar BT1500A looked to be a good choice. In
> early January I
> ordered a Palstar BT1500A. When I received the tuner and put it
> on-line I
> experienced several problems, I had a tremendous amount of rf
> into the
> shack, it actually took out the ptt circuit and the audio
> circuit of my
> FT-897D. The only thing that was changed in the shack was the
> tuner, I
> pulled the MFJ out and slid the Palstar in. In the third week of
> January the
> Palstar was sent to the factory to have it looked at. The tuner
> was repaired
> and returned to me, the packing slip listed a long list of parts
> that had to
> be replaced in the tuner.
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